1. Field of the Invention
The present invention relates to audio coding devices, and more particularly to an audio coding device that encodes speech signals into MPEG Audio Layer-3 (MP3), MPEG2 Advanced Audio Codec (MPEG2-AAC), or other like form.
2. Description of the Related Art
Enhanced coding techniques have been developed and used to store or transmit digital audio signals in highly compressed form. Such audio compression algorithms are standardized as, for example, the Moving Picture Expert Group (MPEG) specifications, which include AAC, or Advanced Audio Codec. AAC, recommended by the International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC) as ISO/IEC 13818-7, achieves both high audio qualities and high compression ratios. AAC is used in various areas, including online distribution of music via mobile phone networks and digital television broadcasting via satellite and terrestrial channels.
The coding algorithm of AAC includes iterative processing operations called inner and outer loops to quantize data within a given bit rate budget. The inner loop quantizes audio data in such a way that a specified bit rate constraint will be satisfied. The outer loop adjusts the common scale factor (CSF) and scale factors (SF) of individual subbands so as to satisfy some conditions for restricting quantization noise within a masking curve, where the term “quantization noise” refers to the difference between dequantized values and original values.
As an example of a conventional audio coding technology, the Japanese Patent Application Publication No. 2002-196792 proposes a technique to determine which frequency bands to encode in an adaptive manner (see, for example, paragraphs Nos. 0022 to 0048 and FIG. 1 of the publication). This technique first determines initial values of a plurality of scale factor bands and thresholds. Out of those scale factor bands, the proposed algorithm selects a maximum scale factor band for determining frequency bands to be coded, based on the psychoacoustic model and the result of a frequency spectrum analysis performed on given input signals.
ISO/IEC AAC standard requires both the above-described inner and outer loops to be executed until they satisfy prescribed conditions, meaning that the quantization processing may be repeated endlessly in those loops.
Conventional algorithms repeat quantization operations until an optimal set of quantization parameters (CSF and SF) is obtained. The problem is slow convergence of iterations and degraded sound quality due to fluctuations in frequency ranges to be encoded.